1. Field of the Invention
This invention relates in general to a unique two element filter embodying an enclosure containing a tandem rotary disk and drum filter, and more specifically is directed to a high efficiency, low residence time system which has both the disk filter and the drum filter in line on the same shaft. Its design permits the filtration media to be changed in less than 10 minutes for different systems or maintenance.
2. Background of the Invention
The main absorbent component of most disposable sanitary products such as diapers and sanitary napkins is the pad or core. Typically the pad is made of wood pulp that has been fiberized by a special mill, designed to handle a special absorbent grade of wood pulp. After the pulp is fiberized, it is drawn out of the mill by an air stream onto a pad former which contains a number of shaped air forming pockets with integral screens.
The screens are generally between 20 and 80 mesh. The pad is formed on the screen with its weight and density controlled by a high velocity air stream. After the pad is formed on the screen, it moves through a process that applies the requisite components to create the complete absorbent product. Proper pad weight and density are extremely important to the disposable sanitary product manufacturer.
Exhaust air from the forming screen contains small amounts of fiberized fluff pulp. Experience has shown that the amount of pulp that comes through the forming screen ranges from 0.4% to 4% of the total amount of fluff that enters the forming chamber. The amount of pulp in the air flow downstream of the former can be increase dramatically when scarfing or pad top trimming is used. All of the this material must be recovered either as waste or more commonly as recycle material fed back to the pad former. It is especially important that it does not enter the ambient atmosphere so as to not violate EPA standards. Also the current prevalence of superabsorbent polymers (SAP) added to the pads can create a health hazard if present in the plant air. Economics are also an important spur to recycling all recovered pulp and SAP.
A number of filters are commercially available for filtering the pulp and SAP out of the pad former's exhaust air. Most of these filters have many disadvantages compared to the instant invention. These disadvantages include high cost limited versatility in terms of materials filtered, complicate internal construction, need for frequent maintenance shutdowns, difficulty in handling super absorbent polymer (SAP) fractions and difficulties in adapting to recycle operation. The last two problems are especially important in the large market of disposable diapers where the fluff pulp that is returned to the process represents a substantial cost savings to the manufacturer.
If the volume of air moving through the forming chamber and its pressure are not consistent pad formation will be poor. If the air volume or pressure is changed, the pad will have different thicknesses, densities and absorbencies and will not meet the user's specifications. By assisting in moving air through the forming screen a properly built and designed filter assists in maintaining correct air volume and pressure in the total system.
The most popular variety of prior art filters use a rotary drum for supporting the filter media. A typical system would include a pulp fiberizing mill followed by the pad former which uses a subsequent suction fan to pull the pulp through the screens. The exhaust of the suction fan leads into the filter and after cleansing the air can be exhausted to atmosphere or recycled to the process. Some filters have internal systems that remove the filter cake and return it to the fiberizer or the former in a separate air stream while others bale the recovered material.
Prior art processes usually utilize one filter for one production line. Drum filters are expensive and take up a large amount of floor space in a plant. There is a need for a filter system which is compact, efficient, versatile, easily adapted to recycle service and inexpensive.
Using one drum filter or other filter configuration for two or more production lines in order to handle larger amounts of fluff has not been an adequate solution to the problem. In this case uniform air volume and pressure will not be maintained if any of the connected production lines cease to operate.
Systems which can handle the higher loads without shutdown problems are expensive and quite complex to operate and maintain.
Another problem found in the rotary drum and other filter systems is that waste pulp has a tendency to accumulate in the bottom and other parts of the filter enclosure. Gravity and dead air spots cause the waste to remain in these corner areas Further, fluff sticks to the accumulated fluff which exacerbates the problem. Frequent shutdowns are required to clean out this fluff accumulation which is expensive and undesirable.
Methods of avoiding fluff accumulation in the comers include baffles in trouble spots to increase air velocities and redirect them, providing a turbulent air stream at the bottom of the filter enclosure, and providing more than one inlet across the bottom of the enclosure in order to create an even more turbulent air flow. However, it has been found that generally these solutions do not solve the fluff accumulation problem.
Fluff accumulation can cause other problems in a filter system. The combustible fluff pulp remaining within the enclosure can act as fuel for a fire or, in some cases, an explosive mixture can be created. Manufacturers have set limits for the amount of fluff per unit volume that they consider a safe amount to be in the enclosure at a given time. This limit is referred to as the lower explosive limit or LEL. Because it is not scientifically determined, as with hydrocarbons, it varies among different manufacturers. Regardless of the limit set accumulated fluff pulp in the enclosure is a genuine safety threat.
To offset this threat some filter manufacturers may provide rupture disks in their equipment to prevent any possible explosions in a rotary drum enclosure from spreading to other parts of a plant. The rupture disk opens when a predetermined pressure or explosive pressure is built up within the enclosure and vents it through a high pressure duct to the outside of the plant. The structure and installation of the explosion vent and its duct work can often be more elaborate and more expensive than the filter. Thus, manufacturers have searched for ways to avoid having to provide explosion vents.